Technological developments in the field of concretes in the last few years have led to the development of innovative cement formulations making it possible to obtain ultra-high-performance concretes in terms in particular of compressive strength. These formulations generally involve the use of further materials in addition to cement and aggregates and/or sand, for example fibers, organic additives or so-called ultrafine particles which are generally smaller than cement grains.
For example the document EP 0518777 describes a mortar composition comprising, apart from Portland cement: sand with a diameter comprised between 80 μm and 1 mm (in particular between 125 and 500 μm), vitreous microsilica with a diameter comprised between 0.1 and 0.5 μm and a water-reducing or plasticizing agent. The microsilica represents only 10 to 30% by weight relative to the cement.
The document WO 95/01316 describes a composition for concrete comprising, besides Portland cement: sand with a diameter of 150 to 400 μm, fine elements with pozzolanic reaction (in particular amorphous silica but also fly ash or blast furnace slag) with a diameter of less than 0.5 μm, a small quantity of metal fibers and optionally ground quartz powder (average size 10 μm) and small quantities of other additives. The amorphous silica can be present in a proportion of 10 to 40% by weight relative to the cement, and the ground quartz powder, when it is used, is typically present in a proportion of 40% by weight relative to the cement. The composition for concrete according to this document therefore requires approximately 900 kg of cement per m3 of concrete.
In the document WO 95/01317, a composition for concrete very similar to the preceding one is disclosed, with exclusively steel wool as metal fibers and amorphous silica as elements with pozzolanic reaction.
The cement compositions described in the document WO 99/23046 are more specifically dedicated to the cementing of wells and, besides a hydraulic binder, comprise: 20 to 35% by weight relative to the binder of microsilica with a grain size comprised between 0.1 and 50 μm, and 20 to 35% by weight relative to the binder of mineral or organic particles with a diameter comprised between 0.5 and 200 μm, as well as a superplasticizer or plasticizer.
The document WO 99/28267 relates to a concrete composition comprising cement and metal fibers as well as: 20 to 60% by weight relative to the cement matrix of sieved or ground sand-type granular elements smaller than 6 mm; elements with pozzolanic reaction smaller than 1 μm; acicular or flaky elements smaller than 1 mm; and a dispersing agent. In the examples, the elements with pozzolanic reaction are constituted by vitreous silica in a proportion of approximately 30% by weight relative to the Portland cement.
In a relatively similar manner, the document WO 99/58468 describes a concrete composition in which there are included at least: a small quantity of organic fibers, granular elements smaller than 2 mm, fine elements with pozzolanic reaction smaller than 20 μm and at least one dispersing agent. In the different examples mentioned, the composition comprises approximately 30% of quartz flour and approximately 30% by weight of silica fume relative to the cement.
These proportions between the different particle size ranges are not fundamentally modified in a later document (WO 01/58826) also disclosing other concrete compositions.
The document EP 0934 915 describes a concrete prepared from cement the grains of which have an average diameter comprised between 3 and 7 μm, to which are added: sand, silica fume with a characteristic diameter of less than 1 μm, an antifoaming agent and a superplasticizer, such that at least three particle size ranges are represented. In the light of the different examples, it is noted that the silica fume is in the minority relative to the cement, the latter typically being present in a proportion of approximately 900 kg per m3 of concrete.
Analysis of the prior art shows:                1) that the optimization of the formulations is specifically directed towards the high or ultra-high-performance concretes and does not apply generally to the concretes in common use; and        2) that all the currently known concretes have a relatively high cement content.        
Thus, even if the standard concretes, which have less good performances in terms of compressive strength than the abovementioned concretes, for example the B25-type concretes (i.e. the compressive strength of which 28 days after mixing is at least 25 MPA) are examined, it is noted that the quantity of cement is typically 260 to 360 kg per m3 of concrete. Moreover, the current European standards do not provide cement levels of less than 260 kg/m3 for concretes in common use.
Now, the methods for producing cement, and more particularly its prime constituent, clinker, give rise to significant carbon dioxide emissions. The production of clinker grains in fact assumes:                a) the preheating and the decarbonation of the raw meal which is obtained by grinding the raw materials, which are in particular limestone and clay; and        b) the firing or clinkering of the meal at a temperature of 1500° C., followed by rapid cooling.        
These two stages produce CO2, on the one hand as a direct product of decarbonation and on the other hand as a by-product of the combustion which is implemented in the firing stage in order to raise the temperature.
The emission level therefore reaches a minimum of approximately 560 kg of CO2 per tonne of binder for a standard B25 concrete (based on 850 kg of CO2 emitted on average per tonne of cement), and it is still greater for a ultra-high performance concrete.
The significant carbon dioxide emissions in the standard methods for producing cement compositions and concrete constitute a major environmental problem, and, in the present context, are subject to significant economic penalties.
A strong need therefore exists for a method making it possible to produce concrete with reduced associated carbon dioxide emissions, said concrete exhibiting satisfactory mechanical properties, in particular equivalent to those of the existing concretes in common use, with a view to its use in the construction industry.